1. FIELD OF THE INVENTION
This invention relates to a process for hardening gelatin and more particularly to a process for hardening gelatin used for photographic light-sensitive materials.
2. DESCRIPTION OF THE PRIOR ART
Gelatin is used as a binder for various photographic light-sensitive materials. For example, gelatin is used as one of the main components constituting a silver halide light-sensitive emulsion layer, a protective layer, a filter layer, an intermediate layer, an antihalation layer, backing layer, a subbing layer on a film support, a baryta layer, and the like.
When light-sensitive materials containing gelatin are processed with various aqueous solutions having different pH's and at different temperatures, layers containing gelatin which has not been treated with a hardening agent swell excessively in the aqueous solutions and are easily damaged since they have poor water resistance. In particular, in aqueous solutions at higher temperatures, e.g., above about 30.degree. C, the gelatin layers may even dissolve and may be leached out in the extreme case.
Various compounds have hitherto been known for effectively hardening gelatin to improve the water resistance, heat-resistance and damage resistance of the gelatin layers. These compounds are well known as hardening agents and are used in the production of photographic light-sensitive materials. Examples of these compounds are aldehyde compounds such as formaldehyde or glutaraldehyde, compounds having reactive halogens as described in U.S. Pat. No. 3,288,775, compounds having reactive ethylenically unsaturated bonds as described in U.S. Pat. No. 3,635,718, aziridine compounds as described in U.S. Pat. No. 3,017,280, epoxy compounds as described in U.S. Pat. No. 3,091,537, halocarboxyaldehydes such as mucochloric acid, dioxanes such as dihydroxydioxane or dichlorodioxane, carboxylic acid esters containing a ##STR2## linkage (hereinafter "C- ester") as described in U.S. Pat. Nos. 3,241,972 and 3,542,558, and Japanese Patent Publication No. 48,896/72, and inorganic hardening agents such as chrome alum, chromium sulfate, aluminum sulfate, potassium alum, ammonium alum or zirconium sulfate.
However, each of the above known hardening agents for gelatin has defects, when used in photographic light-sensitive materials: for example, insufficient hardening effects may be obtained; over a long period, changes in the hardening effect with time due to their slow hardening reaction on gelatin may occur, which is referred to as "after-hardening"; undesirable effects on the properties of photographic light-sensitive materials (particularly, such as an increase in fog, a reduction of light-sensitivity or a change in gradation) may occur; their hardening effects may be lost due to the presence of other photographic additives or the effects of other photographic additives such as color couplers for color light-sensitive materials may be decreased; the production of the compounds used as a hardening agent may be so difficult that they are unsuitable for mass production; the hardening agents themselves may be unstable and not be stored well; or the like.